This proposal focuses on several mutational changes accompanying DNA repair in budding yeast. DNA repair is induced by galactose-regulated expression of the site-specific HO endonuclease, creating a single double-strand break (DSB). One major goal is to understand complex mutations associated by template switching that occur during gene conversion. Two types of repair will be studied: 1. Quasipalindrome mutation formation during gene conversion and 2. Interchromosomal microhomology-mediated template switching during gene conversion. Genetic analysis of helicases and other repair factors will be screened to find proteins that control the level of these two events, along with an exploration of the role of chromatin. A second goal is to understand changes in repeat copy number during gene conversion, motivated by our recent discovery of important differences between DSB break repair and gap repair. In collaboration with Mitch McVey, another member of this Program Project who focuses on DSB repair in fruit flies, we will assess the frequency of abortive gap repair leading to deletions between repeated sequences within the copied region.